Anorexia nervosa (AN) is an increasingly common disorder in adolescent girls, with a prevalence of 0.2 to 1.0% in Western societies. Adolescence is a critical time for bone mineral accretion as more than 90% of peak bone mass is established during this time, and peak bone mass is a major determinant of osteoporosis and fracture risk throughout life. Factors critical to the marked increase in bone mass through puberty include rising levels of estrogen, and estrogen induced increases in growth hormone and insulin like growth factor-1 (IGF-1). Low bone density is a frequent complication of adolescents with AN, and is concerning for increased fracture risk. Mechanisms underlying the low bone density characteristic of AN include profound estrogen deficiency and very low levels of IGF-1. Thus far, there are no established therapies to maximize bone mass accrual and peak bone mass in teenagers with AN, and prevent fractures. Although AN is associated with profound IGF-1 and estrogen deficiency, both of which are critical to bone development, effects of administration of recombinant human rhIGF-1 with estrogen versus estrogen alone has not been investigated in adolescents with this disorder. In addition, the effect of AN on bone microarchitecture, a better indicator of fracture risk than bone density or bone strength, has not been prospectively examined. In the current proposal, effects of administration of rhIGF-1 and estrogen versus estrogen alone will be studied. In the first specific aim we will prospectively investigate the effect of rhIGF-1 and estrogen vs. estrogen alone on bone density and bone turnover in adolescent girls with AN and compare bone accrual to normal adolescents. In the second aim, we will investigate the effects of rhIGF-1 and estrogen on bone microarchitecture and bone strength in adolescents with AN. These studies will be essential in developing strategies to address the profound loss of bone accrual seen in this large population of adolescent girls at high fracture risk.